Imaging Self-Assembled Monolayers Using Lateral Force Microscopy

J. Wilbur, H. Biebuyck, J. MacDonald and G. Whitesides, Department of Chemistry, Harvard University, Cambridge, MA 02138

Patterned SAM across two thicknesses of gold (6µm x 6µm)

Introduction

In this Application Note, we explore the use of lateral force microscopy (LFM) for imaging patterned self-assembled monolayers (SAMs) formed by microcontact printing (PCP) of functionalized alkanethiols, HS(CH2)nY on gold. Since the regions of the SAM are distinguished by only slight differences in molecular structure, their imaging requires sensitivity to the surface chemical composition. This can be done at extremely high resolution using LFM. In addition, this experiment proves the potential that LFM offers to provide the basis for a new approach to experimental tribology, with observation localized on regions less than 100 nm in width.

About the Samples

Patterned SAMs formed by µCP have many applications, including:

* microfabrication

* studies of wetting and nucleation phenomena

* protein and cellular adhesion patterned formation of microcrystals and microcrystal arrays.

LFM image of patterned SAM (50µm x 50µm)

The formation of the SAMs was accomplished by fracturing silicon wafers coated with gold into rectangular slides and washing them with heptane, deionized water and absolute ethanol. They were then dried with a stream of dry N, gas. µCP of alkatheniols using an elastomeric stamp formed patterned SAMs. In initial studies, we patterned gold by PCP to form regions of SAMs terminated by CH3 or COOH. These two functional groups were chosen since they spanned the range of surface polarities and free energies conveniently available with organic surfaces. Previous studies showed that µCP produced homogeneous regions of different chemical functionality separated by sharp boundaries (less than 1OO nm).

LFM image of patterned SAM (17µm x 17µm)

Results

LFM showed contrast between regions ,terminated by CH3, and COOH and accurately reproduced the pattern caused by µCP (Figure 1). The contrast in the image arose from differences in the interactions between the AFM tip and the surface in each region; the tip experienced lower torsion where the patterned SAM terminated in CH3(dark areas) than in regions terminated by COOH (light areas). These differences in torsion in LFM experiments have been interpreted generally as changes in friction between the tip and surface.

LFM image of patterned SAM (5µm x 5µm)

Factors affecting LFM

Plausible factors that influence the torsion felt by the AFM tip include: the interfacial free energy between the tip and the surface adsorbates between the tip and the surface and modifications of interactions due to their presence o sample morphology.

It is preferable, for the sake of convenience, to perform these experiments in ambient laboratory conditions. To prove that contrast between regions of the patterned SAMs imaged in the experiment was not affected by changes in relative humidity, samples were scanned in both ambient and inert environments. Figure 2 shows that, even after a one hour nitrogen flush, the contrast remained unchanged.

To determine the possible role of surface morphology in LFM imaging, a substrate was prepared with two thicknesses of gold (3000 A and 300 A) meeting in a line. µCP of this substrate with hexadecanethiol (darker region in Figure 3), followed by washing with mercaptohexadecanoic acid (brighter region in Figure 3) formed a patterned SAM that extended across areas of both thicknesses. LFM imaging displayed similar contrast between CH3, and COOH groups on areas of the substrate with different morphologies. The strip of CH3 that does not correspond to the pattern of the stamp is most probably due to "wicking" along the groove formed by the step height.

Conclusions

It has been demonstrated that LFM can provide excellent imaging of patterned SAM formed by PCP, showing contrast between regions terminated by different chemical functionalities. The morphology of the underlying surface and the humidity of the environment did not influence this contrast. The use of LFM for this application provides both high contrast and convenience, It also opens a window into tribology on the 100 nm scale. Although we have not yet explored this area quantitatively, the structural flexibility of SAMs, which provides the ability to present a wide range of well-defined functional ,groups at a surface, makes this system attractive for correlating microscopic and macroscopic studies of tribology.

This work was supported in part by the Advanced Research Projects Agency and the Office of Naval Research. The authors would like to acknowledge postdoctoral fellowships from the National Institute of Health, and Merck.

All images were acquired with the TopoMetrix TMX 2000 Discoverer SPM

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